This invention relates to electrostatic printing procedures and more particularly to an improved liquid developer formulation for sustained copy quality.
Processes for forming electrostatic images, existing as electrostatic charge patterns upon a substrate, are well known. In electrostatic printing or copying, a photoconductive imaging surface is first provided with a uniform electrostatic charge, typically by moving the imaging surface past a charge corona at a uniform velocity. The imaging surface is then exposed to an optical image of an original to be copied. This optical image selectively discharges the imaging surface in a pattern to form a latent electrostatic image. In the case of an original bearing dark print on a light background, this latent image consists of substantially undischarged "print" portions corresponding to the graphic matter on the original, admist a "background" portion that has been substantially discharged by exposure to light. The latent image is developed by exposure to oppositely charged, pigmented, toner particles, which deposit on the print portions of the latent image in a pattern corresponding to that of the original.
In liquid developer photocopiers these charged toner particles are dispersed in a dielectric liquid. The dispersion ingredients are carrier liquid, toner particles and charge directors. The charged toner particles in the liquid developer migrate to the oppositely charged "print" portions of the latent image to form a pattern on the photoconductive surface. This pattern, and the corresponding toner particles and residual carrier on the image, and background are then transferred to a sheet to produce a visible image. Any liquid developer remaining on the photoconductive surface after this process is recycled back into the liquid developer reservoir.
Charge directors play an important role in the electrophoretic developing process described above by charging the toner particles in the liquid developer. Stable electrical characteristics of the charged liquid developer are crucial to achieve a high quality image, particularly when a large number of impressions are to be produced without changing the liquid developer solution.
Part of the charge director is adsorbed by the solid toner particles. The overall amount of charge director which remains associated with the solid toner particles via the adsorption mechanism can be determined from an adsorptivity analysis. Details of the analysis techniques will be discussed in a following paragraph.
The quantity of toner particles utilized per photocopy will vary in proportion to the percentage of "print" portions in the original while the amount of carrier liquid transferred increases more slowly as a function of print portion. An original containing a large "print" portion will cause a greater depletion of the toner particles than an original containing a "small" print portion. Thus substantially white originals, i.e. paper containing few covered areas will cause less relative depletion of toner particles. Substantially dark originals, i.e. originals containing numerous graphic images or letter copy, will cause greater relative depletion of toner particles.
The application of liquid developer to the photoconductive surface depletes the overall amount of liquid developer in the developer reservoir. Generally the reservoir is replenished from two separate sources, the first containing carrier liquid and the second containing a highly concentrated dispersion of toner particles in carrier liquid. The charge director is generally added with either the carrier liquid or the toner particle dispersion but not with both. The rate of replenishment of carrier liquid is controlled by monitoring the overall amount of the liquid developer. The rate of replenishment of toner particles is controlled by monitoring the concentration of toner particles in the liquid developer, by optical sensing. Thus, toner particle concentration in the liquid developer dispersion remains relatively constant. However, since charge director is generally added either with the carrier liquid or the concentrated toner particle dispersion but not with both, the charge director concentration in the liquid developer will not remain constant during substantial operation at different copy coverages causing a charge director imbalance in the liquid developer reservoir.
We have discovered that many low quality copies are a result of this charge director imbalance in the liquid developer. The optimum charge director concentration in the liquid developer is usually determined to be the concentration which will achieve high copy quality in copies made from originals having some nominal print area. As previously discussed, the amount of toner particles utilized per photocopy varies in proportion to the "print" area of the original. A large number of originals with small "print" areas (hereinafter "white" copies) will result in very little utilization of toner particles. However, since the total amount of liquid developer utilized per copy varies more slowly with print area, a large number of white copies will deplete the overall amount of liquid developer. In response carrier liquid will be added to the liquid developer reservoir. The amount of carrier liquid added to the reservoir will be much greater than amount of toner particle dispersion since the white copies utilized very few toner particles. If the photocopier is designed so that the charge director is added only with the carrier liquid, the result of a large number of white copies will be an increase in the concentration of the charge director in the liquid developer. The increased concentration of charge director above the optimal value will result in a lower than optimal dispersion of toner particles. The result will be a degradation of copy quality. If the photocopier is designed so that the charge director is added only with the toner particle solution, the result of a large number of white copies will be a decrease in the concentration of charge director in the liquid developer. This decreased concentration will also adversely affect the copy quality.
Similarly, a large number of originals with high "print" areas (hereinafter "black" copies) will cause a degradation of copy quality. Producing the black copies will deplete the number of toner particles in the liquid developer resulting in the addition of concentrated toner particle dispersion to the liquid developer reservoir. If charge director is added with the toner particle dispersion, the concentration of charge director in the liquid developer will be increased. Thus a greater than optimal concentration of charge director will occur resulting in degraded copies. If charge director is added with the carrier liquid, black copies will reduce the concentration of charge director in the liquid developer. This less than optimal concentration of charge director will result in degradation of copy quality.
The optimum solution to the problem of charge director imbalance in the liquid developer would be to separately monitor the charge director concentration in the liquid developer and add charge director to the liquid developer reservoir separately from either the toner particle solution or the carrier liquid. This solution, though possible, would involve the use of costly conductivity or other measurement devices and additional replenishment mechanisms. These devices and mechanisms are impractical in many situations especially as additions to existing photocopier designs.
A simpler solution is needed.
It is accordingly one object of the present invention to provide a solution to the problem of charge director imbalance in liquid developer to provide sustained high quality copies from originals with varying print areas thereby overcoming or otherwise mitigating the problems inherent in photocopying processes known to the art.
Another object of the present invention is to provide a new electrostatic photocopying process whereby charge directors are added to the liquid developer reservoir with both the carrier liquid and the concentrated toner particle dispersion.
A further object of the present invention is to provide a mathematical equation to allow for the calculation of the correct ratio of charge director associated with the carrier liquid and the concentrated toner particle dispersion.
A still further object of the present invention is to provide a new electrostatic photocopying process whereby charge directors are associated with both the carrier liquid and the concentrated toner particle dispersion in a ratio which allows the charge director concentration in the liquid developer to remain relatively constant, regardless of the percentage of print areas on the originals to be copied, thereby producing sustained high quality copies from originals with varying percentages of print areas.
Other objects and advantages of the present invention will become apparent in the following description of the invention.